Such a measuring circuit can be employed, for instance, to detect vibrations. In particular, the measuring object can be constituted by a rotary machine, such as an engine of an airplane or a land based turbine such as a gas or steam turbine, or any other vibrating structure.
A measuring circuit of that type is known from U.S. Pat. No. 6,498,501 B2. The sensing element of this circuit is provided by a piezoelectric transducer. The sensing circuit further comprises two injection capacitors connected in parallel to the piezoelectric transducer. Both injection capacitors are connected to the injection line such that the auxiliary test signal can be injected from the output of the signal injector into the sensing circuit by means of the signal injection capacitors. The test signal is then transmitted from the injection points to the evaluation circuit via a respective transmission line. This enables the measuring circuit to evaluate the quality of the measuring circuit not only during operation of the monitored vibration machine but also when the vibration machine is at rest. In this way, a permanent surveillance of the measuring circuit can be achieved.
A disadvantage of this measuring circuit is that its internal built-in test equipment (BITE) is limited to the monitoring of a proper functioning of the transmission lines and of a proper connectivity to external components, since only those are employed to deliver the test signal to the evaluation circuit. But the sensing element itself cannot be tested as the auxiliary test signal is passed around the terminals of the sensing element by the signal injection capacitors. The quality of the sensing element, however, is of crucial importance for the reliability of the measuring circuit.
Another disadvantage is the need of provision of signal injection capacitors. In general, such additional components of a desired high reliability lead to increased production costs. On the other hand, additional components have intrinsically a certain probability of failure and will therefore contribute to possible sources of error in the measuring circuit.
It is an object of the present invention to remedy at least one of the above mentioned disadvantages and to provide the initially addressed measuring circuit with an improved BITE functionality. In particular, possible sources of error existing in current measuring circuits shall be made better identifiable or reduced or eliminated by the newly proposed circuit design according to the invention.